The field of the invention relates to engine air/fuel control systems including systems which rapidly warm the catalytic converter.
U.S. Pat. No. 5,211,011 discloses a system in which the fuel delivered to an engine is alternated between rich and lean values while ignition timing is retarded to more rapidly heat the catalytic converter.
The inventors herein have recognized numerous problems with the above approach. For example, fuel modulation, under certain operating conditions, may cause an excessively lean shift in engine air/fuel ratio resulting in rough engine operation. Still another problem recognized by the inventors is that the air/fuel ratio may drift lean and cause misfires, or may drift rich causing excessive emissions.